Glass material is used for top-sheet members for photovoltaic cells. Glass top-sheet members are excellent in dimensional stability and are flame retardant, but there is a limit to make them lightweight, and photovoltaic modules may be damaged when impact is exerted thereon. Thus, it has been studied to replace conventional glass top-sheet members with top-sheet members that are made from a composite using resin as the matrix material.
When top-sheet members are replaced with a composite, it is necessary to keep translucency characteristics of the composite at substantially the same level as that of the matrix resin so that the photoelectric conversion efficiency of photovoltaic cells is maintained. In addition, when a composite is used as a top sheet of a photovoltaic cell the top sheet, an encapsulant and a photovoltaic cell need to be thermally laminated. However, since an approximate temperature of 150° C. is added during the lamination process, it is necessary to suppress thermal expansion and to secure stiffness of the top sheet. Having low-quality properties is not preferred, since photovoltaic cells may warp or be damaged.
To obtain resin material with excellent translucency characteristics, a method is known such as dispersing translucent particles, fibers or the like in translucent matrix resin so as to adjust differences in refractive indices between the matrix resin and particles.
For example, patent publication 1 proposes a method for dispersing glass fiber or fine particles in translucent resin so as to achieve the difference in their refractive indices at 0.01 or lower. However, cured resin is used as the translucent resin in the method. When a composite is formed prior to curing the resin, sedimentation of glass material occurs due to the difference in specific gravities between the resin and glass material, and it is difficult to have homogeneously dispersed glass in the cured material when forming a thick molded article. It was especially difficult using fine-particle or flake-type glass material to produce a thick molded article in which glass material is homogeneously dispersed.
Also, patent publication 2 proposes a method for dispersing glass flakes with an average particle size of 5 to 50 μm in thermoplastic resin. However, since the average particle size of the glass in the method is small, the obtained molded article does not have sufficient translucency characteristics.
Patent publication 3 proposes a method for dispersing glass beads with an average particle size of 1 to 10 μm in thermoplastic resin. However, since the glass used in the method is sphere-shaped with a small average particle size, multiple scattering tends to occur and translucency characteristics may decrease in the molded article to be obtained.
Patent publication 4 discloses a composition containing rubber-reinforced polystyrene resin serving as a matrix and glass flakes or mica dispersed therein. However, the rubber dispersed in the matrix causes higher thermal expansion and reduced stiffness. Thus, it is not sufficient for use as a top sheet of a photovoltaic cell. In addition, regarding the materials disclosed in examples of patent publication 4, the total light transmission is approximately 70% at the maximum, and translucency characteristics are not sufficient.
In addition, patent publication 5 proposes a composite of an aromatic polycarbonate resin and glass flakes or glass fiber whose refractive index has a difference of 0.015 or lower from that of the aromatic polycarbonate resin. Such a composition has sufficient stiffness and translucency properties, but its weathering resistance is not sufficient because of the aromatic polycarbonate used as matrix material.